What are the Compositions of Magmas Erupting on Io?
A.S. McEwen, L.P. Keszthelyi (LPL), Galileo SSI Team
Recent Earth-based and Galileo observations have shown that high-
temperature hot spots are common on Io. The highest temperatures are
well above 500 K, too hot for elemental sulfur alone, so Io's volcanism
is probably driven by silicate eruptions. Two short-lived events
observed from the ground (Veeder et al., 1994, JGR 99, 17,095; Spencer
et al., GRL, in press) require temperatures of 1500 K or hotter. Because
of rapid radiative cooling, even within a vigorous fire fountain, remote
temperature measurements are usually at least 200 K lower than the
actual eruption temperature. A magma temperature exceeding 1700 K is
consistent with ultramafic melts, not basalts which are typically
erupted at less than 1400 K. Were these high-temperature events
fortuitous observations of unusual magma compositions, or are many
eruptions on Io driven by very high-temperature magma? Galileo SSI has
observed Io during 5 eclipses in the first 8 orbits, including
observations through the broad-band clear filter ( 0.4-1.0 microns) and
color filters with effective wavelengths near 0.76, 0.89, and 0.99
microns. Preliminary analyses of these data indicate that temperatures
commonly exceed 1300 K, and sometimes exceed 1700 K. Hence, it now
appears possible that much of Io's volcanism is driven by ultramafic
magmas. Most ultramafic melts on Earth are primitive, undifferentiated
magmas, but Io is likely to be very highly differentiated (Keszthelyi
and McEwen, submitted). The differentiation of Io may have produced a
depleted Mg-rich mantle which evolves toward a composition of pure
forsterite ( ), with a liquidus temperature near 2160 K.
Could Mg-rich melts reach Io's surface? A dense melt can rise through
a low density differentiated crust given deep magma chambers (> 100 km),
a source of volatiles, or some other process to produce overpressurization. Other observations from SSI that may support this
hypothesis are that: (1) the hot spots are invariably confined to
relatively low elevations on Io, especially caldera floors; and (2) the
low-albedo materials (closely associated with hot spots) do not have a
detectable 1-micron absorption band, expected from Fe-rich silicates
but not Fe-poor silicates such as forsterite.